Biology and Conservation of North American Tortoises

1 Morphology, Taxonomy, and Distribution of North American Tortoises: An Evolutionary Perspective

Comparative skeletal morphology has long played a central role in deciphering the evolutionary relationships of land tortoises (e.g., Williams 1950a, Loveridge and Williams 1957), including the North American gopher tortoises (Auffenberg 1974, 1976; Bramble 1971, 1982; Crumly 1994). Thus, skeletal morphology is emphasized throughout this chapter because it reflects some of the most profound evolutionary patterns within gopher tortoises, contributes directly to the fossil record (chapter 2), and has been the focus of phylogenetic analyses (e.g., Bramble 1982, Crumly 1994) central to the debate over how best to classify these tortoises. Some skeletal features, including those used below to define various taxonomic groups, will be unfamiliar to the general reader. Chapter 2 offers an illustrated introduction to some of the most important features related to shell morphology. Informative illustrations and discussions of other skeletal features can be found in selected publications (skulls—Gaffney 1979, Bramble 1982; limbs and vertebrae—Bramble 1982, Crumly 1994). sYstematiCs anD taXononY Murphy et al. (2011; chapter 3) summarize the complex and sometimes confused systematic and nomenclatorial histories of the North American gopher tortoises. For the purposes of this chapter we have elected to recognize the two clear species groups within gopher tortoises as the genera Xerobates and Gopherus. The same classification is adopted by Franz (chapter 2), but other contributors to this volume employ Xerobates only as an informal subgroup within the genus Gopherus or simply assign all species (living and extinct) to Gopherus. We have also used the taxon Oligopherus Hutchison 1996 as a “placeholder” for the most widely accepted ancestral gopher tortoises, whose proper taxonomic assignment is uncertain at this moment. Evidence in support of our classification of gopher tortoises is discussed in parts two and three of this chapter. 1 Modern gopher tortoises are the remains of a once abundant and diverse North American tortoise fauna (Williams 1950a, Auffenberg 1974, Bramble 1971). The group is endemic to North America and has an especially rich fossil record. As the name suggests, gopher tortoises are notable for their digging abilities. Although digging is reported for some Old World tortoises (e.g., Centrochelys sulcata Miller 1779), the gopher tortoises are unique in representing the only known example of true fossorial specialization among terrestrial turtles. Fossorial vertebrates are those specialized for digging and spending appreciable time underground (Hildebrand 1985). They also exhibit associated modifications—i.e., morphological, physiological, and behavioral (Gans 1974). As this and other chapters of this volume make clear, and despite very significant advances in recent years, important questions remain as to the evolution, biogeographic and life histories, and appropriate taxonomic treatment of gopher tortoises. Unfortunately , unless efforts to stem the severe threats facing the remaining wild populations of these tortoises succeed (see chapters 17, 18), we may never fully secure the answers. This chapter consists of several parts. We provide an overview of the taxonomy, distribution, and important morphological distinctions among the five living species of gopher tortoise. We then highlight key morphological distinctions that characterize the two primary lineages of extant gopher tortoises and discuss how they reflect differences in the behavior and ecology of the two groups. We next consider controversies regarding the major evolutionary patterns within gopher tortoises and how to best classify these tortoises in light of those patterns. In that context, we note important evolutionary patterns (parallelism, convergence) and mechanisms (paedomorphosis) that have been largely overlooked in these debates. Lastly, we list some unresolved questions concerning the form, function, behavior, and evolution of these unique tortoises and ones that we hope will be the targets of future research. Dennis M. Bramble J. Howard Hutchison Morphology, Taxonomy, and Distribution of North American Tortoises An Evolutionary Perspective Dennis M. Bramble and J. Howard Hutchison 2 Dennis M. Bramble and J. Howard Hutchison but inner ear reduced and containing a small saccular otolith ; prootic and opisthotic bones narrowly exposed on surface of skull; tympanic cavity large. Tomial ridge of upper jaw high; well-developed postmaxillary process; postorbital bar narrowed; posterior pterygoid well ossified; no vomerbasisphenoid contact. Vertebrae: Essentially as in Oligopherus. Manus: moderately broadened, digitigrade, with 3 subradial bones (carpals 1, 2 + medial centrale); pisiform typically present ; proximal phalanges disc-like, lacking retroarticular processes ; ungual phalanges of moderate size, round in section. Mesocarpal joint well developed. Shell: Vertebral scutes as in Oligopherus to wider than long, borders straight; vertebral 1 narrower than 2. Gular projection usually pronounced (esp. in older males) and often divided at tip. Frequently having multiple inguinal scutes. genus gopherus rafinesque 1832 Synonyms: Bysmachelys Johnston...